The films used in these packages comprise at least three layers, namely an outer layer intended to be complexed thereafter, a sealing and separable layer, defining all or part of the internal surface of the package, and between them a hot-melt adhesive layer that joins them together.
In this type of package, the adhesive plays an essential role insofar as, once the package has been opened by the end user, the quality of the reclosure and of the reopening will depend on the performance of the adhesive, which is of course a pressure-sensitive adhesive, that is to say an adhesive that adheres simply by manual pressure. These adhesives are usually known as PSAs (pressure-sensitive adhesives).
Various techniques or processes may be employed for obtaining multilayer films such as those used in the “repositionable” packaging field; mention may be made of lamination-coating, coextrusion-coating, flat coextrusion and coextrusion-blowing of a film bubble (Ref: Plastics Extrusion Technology, Friedhelm Hensen Editor, Carl Hanser Verlag, Munich, 1988).
Patent EP 0 160 975 (NYFFELER, CORTI AG) discloses a hot- or cold-sealable multilayer sheet for recloseable packages. The structure disclosed comprises in particular “a permanently tacky sheet consisting of a pressure-sensitive adhesive, being composed for example of hot-melt adhesive products (hot-melt adhesives) based on ethylene/vinyl acetate (EVA) copolymers, styrene/butadiene/styrene block copolymers (SBS), styrene/isoprene/styrene block copolymers (SIS), or polyacrylates, or the like”. The adhesive formulations are described very briefly and the claimed structure is obtained by optimizing the process and not the adhesive. The process used requires a priori a large number of steps.
U.S. Pat. No. 5,089,320 (JAMES RIVER) relates to a multilayer film for recloseable packaging obtained by coextrusion or coextrusion coating. That patent describes more precisely the adhesives employed, namely PSAs comprising 40 to 80 wt % of a thermoplastic elastomer, such as a styrene/isoprene/styrene block copolymer (SIS) or a styrene/butadiene/styrene block copolymer (SBS) such as KRATON® D-1107 (from Kraton Polymers) and 20 to 60% of a tackifier such as an oil-based resin (ESCOREZ® 1310 LC from Exxon Mobil Chemical). That patent mentions opening forces of 4 to 9 N/cm and reopening forces of 1 to 7 N/cm, the examples cited showing only low reopening values of around 1 to 2 N/cm.
GB patent 2 319 746 (DOLPHIN PACKAGING MATERIALS LIMITED) discloses two films, one serving as a container after thermoforming, the other serving as a lid intended to be heat-sealed around the outline of the first film, and containing a layer of PSA-type adhesive consisting of an elastomer and a tackifying resin such as M3062 from Ato Findley Inc., the sealing layer for the two films being an ionomer, for example a SURLYN® from DuPont, an EMA or an EMA/EBA copolymer. Again, this patent gives a very general description of the adhesive, and the structure is optimized by the process, it being in particular recommended to use ionomer resins as sealing layer, which makes the cost of the package high.
Patent application EP 1 053 953 (SOPLARIL) discloses a recloseable package, both rigid and flexible, both gas-filled and under vacuum, obtained from a coextruded structure. The performance characteristics of the package are obtained here either by optimizing the process, in particular by a bubble reclosure technique with the adhesive as inner layer of the bubble, or by modifying the adhesive, in particular by the addition of fillers or processing aids, which therefore reduces the pressure-sensitive adhesive properties required for reclosure, allowing the contents of the packages to be suitably protected, this drawback being characterized by too low a reopening force.
In short, the multilayer films used in the various packaging systems are not entirely satisfactory, either because the force to be applied to open them is too high or because the reclosure is not of sufficient quality, or else because the desired structure requires a process difficult to implement and incurring a cost burden or a lack of productivity.
It is necessary to have an adhesive that, within the multilayer film, gives the final package produced properties allowing easy first opening and satisfactory reclosing/reopening cycles, without involving complicated process modifications. It is therefore necessary, on the one hand, to have an adhesive exhibiting good processing properties during manufacture of a multilayer film by coextrusion and, on the other hand, the layer of this same adhesive, present within the film, gives the final package properties allowing easy first opening and satisfactory reclosing/reopening cycles. The term “easy first opening” is understood to mean the application of a force of less than or equal to 15 N/cm, with a propagation force, once the package has been opened, of around 5 to 11 N/cm. The adhesive must also make it possible to obtain an easy self-adhering reclosure of the packages, that is to say easy manual repositioning of the two parts of the film after a series of closing/reopening operations, about ten, for reopening forces of greater than 2 N/cm, preferably greater than or equal to 4 N/cm.
A hot-melt adhesive may also be introduced into a multilayer structure using a process that is in tune with economic and industrial requirements. Other techniques of the lamination-coating and coextrusion-coating type have the drawback either of adding an additional step to the process for manufacturing complex films, or of exhibiting drawability or compatibility problems that at the very least may limit the production rates. The incorporation by coextrusion of the adhesive according to the invention as a thermoplastic advantageously makes it possible to meet these requirements. One particularly suitable process for high production rates is the coextrusion-blowing of a bubble.
In the field of adhesives, two types of failure in peel may be distinguished—adhesive failure, which occurs at the adhesive/substrate interface, and cohesive failure that occurs within the actual layer of adhesive, each of the substrates bearing a fraction of the adhesive. A cohesive-type failure considerably improves the “recloseability” property of the package. However, this intrinsic property of the adhesive is in general inconsistent with the viscosity and hot cohesion properties required for coextrusion.
It is therefore necessary to be able to have hot-melt adhesives having both a high hot viscosity (at a temperature of at least 130° C.), allowing coextrusion, and an adjusted cold cohesion (from −20 to +40° C.), allowing predominantly cohesive failure during the first opening, these two properties competing with each other and being difficult to obtain at the same time.
The term “high-viscosity adhesive” is understood to mean an adhesive whose viscosity, measured using the conventional Brookfield method in Thermosel sample chambers according to the ASTM D-3236 standard, is greater than 200,000 mPa·s at a temperature of 200° C.